Colossus and D-Day

65 years ago today General Dwight D. Eisenhower and his staff met to discuss the Normandy landings, or D-Day. The landings had been planned for some time and their success depended on good weather for the crossing and landing, and minimal resistance from German troops so that the Allies could gain a foothold.

Weather conditions had been too poor for a landing in early June 1944, but chief meteorologist James Martin Stagg forecast an improvement on 6th June. This weather forecast is usually cited as the deciding factor in Eisenhower’s decision to set D-Day for 6th June. However, Eisenhower is said to have received another piece of information during that meeting which was just as crucial, and he had the skill and inventiveness of the Post Office Research Station, Dollis Hill to thank for it.

Post Office engineers re-wire a telephone exchange after an air raid. Post Office telephone engineers developed the first programmable electronic computer during the 2nd World War.

Before the war Post Office engineer Tommy Flowers and his team at Dollis Hill had worked in switching electronics, exploring the possibilities for electronic telephone exchanges. But by the early 1940s they were helping the British code-breaking team at Bletchley Park. Colossus, later recognised as the world’s first programmable electronic computer, was their greatest achievement.

Colossus was primarily developed to decipher the Nazi Lorenz codes, high-level encryptions used by senior personnel, rather than the more famous Enigma codes used by field units. Computer technology was in its infancy in the 1940s and when in early 1943 Flowers proposed the machine, which would run on 1800 valves (vacuum tubes), there was great scepticism that it would work as until that point the most complicated electronic device had used about 150 valves.

But by December 1943 Colossus Mark 1 was working and it was soon moved to Bletchley Park, where it was able to break German codes within hours. An improved version, Colossus Mark 2, using 2400 valves, was unveiled on 1st June 1944, four days before Eisenhower made his decision about D-Day.

An essay by Flowers published in Colossus: The Secrets of Bletchley Park’s Code-breaking Computers describes the crucial meeting between General Eisenhower and his staff held on 5th June 1944. During that meeting a note summarising a recent Colossus decryption was handed to Eisenhower. It confirmed that Hitler was aware of troop build-ups in southern England, but would not be sending extra troops to Normandy as he was certain that Allied preparations were a hoax. This information was said to have convinced Eisenhower that the Normandy landings should take place the next day.

But whether it was the weather forecast or the Colossus decryption which tipped the balance in favour of 6th June, Flowers and the Post Office Research Station team made a remarkable advance in computer technology. By the end of the war 10 Colossus Mark 2 computers were in use at Bletchley Park, providing vital information to Allies forces, which certainly reduced the length of the war. After the war Flowers and his team returned to their work in switching, later pioneering all-electronic telephone exchanges. Their ingenuity was only recognised in the 1970s when restrictions on the Colossus project under the Official Secrets Act were lifted.